CS8602 despite guard call in generic method

[skrysm]

Say I have this (simplified) code:

public static class Guard
{
    [ContractAnnotation("value:null => halt")]
    public static void IsNotNull<T>([ValidatedNotNull] T value, string paramName)
    {
        if (value is null)
        {
            throw new ArgumentNullException(paramName);
        }
    }
}

public static class SecureStringHelper
{
    public static string ToStringSecure<T>(T value)
    {
        Guard.IsNotNull(value, nameof(value));

        if (value is SecureString)
        {
            return "<masked>";
        }

        return value.ToString(); // gives: CS8602: Dereference of a possibly null reference.
    }
}

In the method SecureStringHelper.ToStringSecure() I'm getting a CS8602 (Dereference of a possibly null reference.) for value.ToString().

This is because the compiler doesn't know that Guard.IsNotNull(value, nameof(value)); has already verified that value is not null.

Is there a way to communicate this behavior to the C# compiler?

Other code flow analysis tools can already do this:

• For ReSharper, I provide [ContractAnnotation("value:null => halt")]
• For FxCop - specifically CA1062 - I provide the [ValidatedNotNull] attribute.

I know I could write value!.ToString() but I'd rather have the compiler pick up on this automatically.

[333fred]

Yes, via the NotNull attribute on the value parameter. See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/nullable-analysis#specify-post-conditions-maybenull-and-notnull

[skrysm]

Just for anyone else looking for this: the [NotNull] attribute must be applied to the value parameter in IsNotNull().